1. Field of the Invention
The invention relates generally to navigation systems and more specifically to navigation receivers that operate with signals received from orbiting Russian navigation satellites.
2. Description of the Prior Art
The USSR, and now Russia, have deployed and support a satellite-based navigation system that is very similar to the global positioning system (GPS) erected by the United States government. The Russian system is called the global orbiting navigation satellite system (GLONASS) and transmits two ranging codes on two separate microwave frequency bands, e.g., "L1" around 1600 MHz and "L2" around 1250 MHz. GPS uses 1575.42 MHz and 1227.6 MHz, respectively. A coarse acquisition (C/A) code is transmitted on L1 and a precision (P) code is transmitted on both L1 and L2, for both systems.
Frequency division multiple access (FDMA) is used by GLONASS for discriminating between each of the orbiting satellite's signals. The GPS uses code division multiple access (CDMA) to sort out signals. There are also fundamental differences in the structure and content of the almanacs and ephemeris information relating to satellite orbit parameters between the two systems. Therefore a GPS navigation receiver is totally incompatible with GLONASS operation and modifications of major fundamental elements of a GPS receiver would be required to adapt it to GLONASS use.
Unfortunately, the frequency division scheme used by GLONASS required a very broad part of the radio spectrum, and certain in-band frequencies coincide with natural radio emissions from celestial objects in the universe. Thus GLONASS satellite transmissions were disturbing radio astronomy efforts around the world. In response, the number of channels used by GLONASS were cut in half. The way that this was accomplished was to arrange for pairs of satellites orbiting on opposite sides of the earth to share the same frequency. No one earth-bound receiver could possibly receive both transmissions simultaneously. A challenge is thus presented in GLONASS navigation receiver design to identify particular satellites, especially those sharing the same frequency, since none send any unique identifying data in their transmissions.
Further compounding GLONASS navigation receiver design is the fact that the particulars concerning the P-code transmissions is largely undocumented publicly, such information is kept classified by the Russian government. The details of the GLONASS L1 C/A code are officially available and researchers in the West have managed to decrypt the GLONASS P-code. It remains to be seen whether the GLONASS P-code can be encrypted. Further information on this subject is included in, "Global Satellite Navigation System GLONASS Interface Control Document (Second Wording)", Russian Institute of Space Device Engineering/Research and Production Association of Applied Mechanics, 1991. Also see, "The USSR's GLONASS P-Code-Determination and Initial Results", by Gary R. Lennen, Proceedings of ION GPS-89, The Second International Technical Meeting of the Satellite Division of the Institute of Navigation, Colorado Springs, Colo., Sep. 27-29, 1989.
The GLONASS is more appealing for systems requiring superior point positioning accuracy because there appears to be no deliberate dithering function similar to selective availability (SA) connected with the GPS. It can also provide integrity enhancements to GPS navigation systems with independent system measurements. Such independent corroboration of measurements can be very valuable in automatic aircraft landing and geodetic surveying applications.